Test of lepton flavor universality in B^± → K^±µ^+µ^− and B^± → K^±e^+e^− decays in proton-proton collisions at √ s = 13 TeV

Abstract

A test of lepton flavor universality in B^± → K^±µ^+µ^− and B^± → K^±e^+e^− decays, as well as a measurement of differential and integrated branching fractions of a nonresonant B± → K^±µ^+µ^− decay are presented. The analysis is made possible by a dedicated data set of proton-proton collisions at √s = 13TeV recorded in 2018, by the CMS experiment at the LHC, using a special high-rate data stream designed for collecting about 10 billion unbiased b hadron decays. The ratio of the branching fractions B(B^± → K^±µ^+µ^−) to B(B^± → K^±e^+e^−) is determined from the measured double ratio R(K) of these decays to the respective branching fractions of the B^± → J/ψK^± with J/ψ → µ^+µ^− and e^+e^− decays, which allow for significant cancellation of systematic uncertainties. The ratio R(K) is measured in the range 1.1 < q^2 < 6.0 GeV^2, where q is the invariant mass of the lepton pair, and is found to be R(K) = 0.78^8{+0.47}_{−0.23}, in agreement with the standard model expectation R(K) ≈ 1. This measurement is limited by the statistical precision of the electron channel. The integrated branching fraction in the same q^2 range, B(B^± → K^±µ^+µ^−) = (12.42 ± 0.68) × 10^{−8}, is consistent with the present world-average value and has a comparable precision.